The use of sample solution nebulizer systems to introduce liquid samples into sample analysis systems is well known. Sample solution nebulization is typically accomplished by known mechanical, pneumatic or ultrasonic nebulizer means for instance, and available sample analysis systems include Inductively Coupled Plasma (ICP), other plasma based systems, and mass spectrometers.
Typically, sample solution nebulization is carried out in an aerosol chamber at a location remote from a sample analysis system, and nebulized sample droplets must be transported to the location of the sample analysis system by way of a connection means. A common problem which occures during use is that nebulized sample is lost by adherence to the internal walls of the aerosol chamber and connection means between the output of the sample nebulizer system and the input to the sample analysis system. Additionally, the aerosol chamber and connecting means volume must be filled with nebulized sample to cause nebulized sample to eject from said connection means into the remotely located sample analysis system. A relatively larger amount of nebulized sample must then be prepared than would be the case if the sample nebulizer system had no aerosol chamber and was situated in closer proximity to the sample analysis system. System sensitivity is, as a result, adversely affected and tedious, time consuming, system flushing procedures are often required to prevent sample carry-over from one analysis procedure from contaminating subsequent analysis procedure results. It would then, be very beneficial if a sample nebulizer system which did not require an aerosol chamber and which could be positioned closely adjacent to sample analysis systems were available.
In view of the identified problems, Fassel et al., designed a "micro nebulizer" system and obtained a Patent thereon in 1986, said Patent being No. 4,575,609. The Fassel et al. teachings are that the micro nebulizer should be inserted directly into a standard torch of the type used in Inductively Coupled Plasma sample analysis procedures, in which standard torch, during use, a plasma is formed. The micro nebulizer is designed to perform sample solution nebulization directly. That is, the aerosol chamber internal volume and connection means internal volume, between the sample nebulizer system and a remotely located sample analysis system, are eliminated.
The Fassel et al. invention assumes the presence of a first tube, which first tube is essentially the sample injector tube of a inductively coupled plasma standard torch. Briefly, to aid with understanding, said standard torch is comprised of a series of elongated concentric tubes, which concentric tubes are typically, but not necessarily, made of quartz. The centermost tube is typically termed the sample injector tube. It is typically circumscribed by an intermediate tube, which intermediate tube is typically circumscribed by an outer tube. One can visualize the torch system in side elevation, from a position perpendicularly removed therefrom, with the longitudinal dimensions of the various elongated tubes projecting vertically upward from an underlying horizontal surface. Sample particles from a typical sample nebulizing system are typically injected vertically into the sample injector tube of the standard torch from a sample access port at the vertically lower aspect thereof, and caused to flow through said sample injector tube to the upper aspect thereof under the influence of a pressure gradient, whereat they are ejected into the space above said upper aspect of the sample injector tube, which space is typically within the volume circumscribed by the outer tube of the standard torch system, in which space a plasma is typically created during use. As well, typically tangentially injected gas flows are entered into the annular spaces between the outer surface of the sample injector tube and the inner surface of the intermediate tube, and between the outer surface of the intermediate tube and the inner surface of the outer tube. (Note, tangential is to be understood to mean that a gas flow follows a spiral-like upward locus path from its point of entry to the standard torch). The typically tangentially injected gas flows are entered by way of intermediate and outer ports also present in the torch. Said typically tangentially injected gas flows serve to shield the various tubes which they contact from the intense temperatures and heat formed by creation of a plasma in the upper aspects of the torch, and to some extent aid sample flow into the plasma associated area.
The Fassel et al. invention teaches that rather than enter a previously, distally, nebulized sample to the sample access port of a standard torch, a micro nebulizer should be entered into the sample injector tube and positioned so that the upper aspect thereof is at an essentially equal vertical level with the upper aspect of the sample injector tube of the standard torch, into which the micro nebulizer is inserted. Sample solution is then entered into the micro nebulizer via a sample delivery inner tube, directly, without any prior sample nebulization being performed thereon. The Fassel et al. micro nebulizer is designed to cause sample solution entered thereto, to eject from the upper aspect of the micro nebulizer and thereby become nebulized. The upper aspect of the sample delivery inner tube thereof, is positioned at essentially the same vertical level as the upper aspect of the sample injector tube of the standard torch, hence, is located very near the position at which a plasma can be created for use in analysis of the ejected nebulized sample. It will be appreciated that the only nebulizer internal volume which exists is that within the micro nebulizer and the associated connection means thereto from the source of sample solution. Said internal volume is typically on the order of five (5) microliters and is orders of magnitude smaller than the internal volume associated with the sample injector tube of a standard torch and the connecting means thereto from a remotely located conventional sample solution nebulizer system.
To better understand the Fassel et al. micro nebulizer it is necessary to better describe the system thereof. Basically, the Fassel et al. micro nebulizer is comprised of an inner tube and an outer tube, which inner tube is concentrically circumscribed by said outer tube. The two concentric tubes are oriented vertically and placed into the first tube, which first tube can be thought of as the sample injector tube of a standard torch as described above. A sample solution of can be entered into the micro nebulizer at the lower aspect of the inner tube thereof and caused, under the influence of a pressure gradient, (typically 100 to 1000 psi), to flow vertically upward and eject from the upper aspect of the inner tube of the micro nebulizer. Sample solution velocities on the order of one-hundred (100) meters-per-second are common. In addition, a gas flow can be entered into the annular space between the outer surface of the inner tube and the inner surface of the outer tube of the micro nebulizer, which gas flow interacts with the sample solution flow at the point of its ejection from the inner tube of the micro nebulizer, thereby causing said sample solution to be nebulized by essentially pneumatic means. An additional gas flow can be injected into the annular space which results between the outer surface of the outer tube of the micro nebulizer and the inner surface of the sample injector tube of the standard torch into which the Fassel et al. micro nebulizer is inserted. Said additional gas flow can also aid with the sample solution nebulization effect. The nebulized sample solution then immediately injects into the space in the standard torch in which a plasma can be created. The disclosure of the Fassel et al. Patent teaches that a support tube should be epoxied to the outer surface of the outer tube of the micro nebulizer, along some portion thereof which is inside the first tube, (i.e. sample injector tube of the standard torch), during use, apparently to protect the outer and inner tubes thereof against being crushed when inserted into the sample injector tube of the standard torch, and to aid with a firm fit within the sample injector tube of the standard torch into which the micro nebulizer is inserted. The Fassel et al. disclosure teachings also indicate that the outer and inner tubes of the micro nebulizer should be attached to the standard torch by way of a fixed fitting, and that the upper aspect of the inner tube of the micro nebulizer should be positioned vertically at a level below the upper aspect of the sample injector tube of the standard torch. The drawings of Fassel et al. show that the upper aspect of said inner tube of the micro nebulizer is also placed vertically below the upper aspect of the outer tube of the micro nebulizer and that said outer tube of the micro nebulizer and the sample injector tube of the standard torch are tapered inwardly at their upper aspects. In use, it has been found, that the Fassel et al. system as described above, particularly when used with high solids content sample solutions, becomes clogged at the upper aspect thereof. This results in the necessity that the micro nebulizer be cleaned often, which cleaning is difficult to perform and often leads to breakage of the micro nebulizer. It has also been discovered that the upper aspect of the inner tube of the Fassel et al. system is difficult to position inside the outer tube of the Fassel et al. system, and that the Fassel et al. system tends dislodge from the point at which it is secured inside the standard torch sample injector tube at the lower aspect of said sample injector tube, when relatively high pressure gas flow is entered into the annular space between the outer surface of the outer tube of the micro nebulizer and the inner surface of the sample injector tube of the standard torch. It is emphasised that the securing of the micro nebulizer to the inside of the sample injector tube of the standard torch is by way of a fitting, through which fittinq is run the outer and inner tubes of the micro nebulizer.
The inventor in the present Application has found that great utility would result from major changes to the Fassel et al. system, which changes would provide means which allow a user thereof to:
1. easily access the inner portion of the upper aspect of the micro nebulizer; and PA1 2. easily insert the inner tube of the micro nebulizer and adjust the vertical location of the upper aspect thereof independent from any interaction with the outer tube thereof. PA1 1. Nebulizing a sample solution to form sample solution droplets. PA1 2. Desolvating the resulting nebulized sample solution droplets and removal of the solvent. PA1 3. Transporting the sample through the nebulizing system, desolvation and solvent removal systems into a sample analysis system. PA1 4. Doing the above with varying degrees of success as regards use with either water or organic solvents, minimizing sample carry-over from one analysis procedure to a subsequent analysis procedure and achieving long term stability of operation.
Other improvements in the Fassel et al. system would result from use of a protective sleeve around at least a portion of the extent of the inner tube thereof, use of hydrofloric acid resistant nonmetalic materials in the construction thereof, and use of a unibody design for the basic portion of the micro nebulizer, which unibody design allows for connections at the lower, middle and upper vertical aspects thereof. The connection at the upper aspect thereof being to allow easy access and cleaning of accumulated sample solids, the connection at the lower aspect thereof being to allow inner tube upper aspect vertical level positioning, and the connection at the middle thereof being to allow attachment to a source of gas to cause a flow thereof into the annular space between the outer surface of the inner tube of the micronebulizer and the inner surface of outer tube thereof, which outer tube thereof would be formed by the unibody design of the micro nebulizer. The use of only nonmetalic materials is proposed to prevent untoward interaction with plasma energy which is common when metals are present near a plasma, and the use of hydrofloric resistant materials, (e.g. polyimides), is proposed to allow use of hydrofloric acid as a sample solvent.
Another very recent patent, No. 4,990,740 to Meyer, recognizes the benefits and problems associated with the Fassel et al. micronebulizer, and teaches an Intraspray ICP Torch which serves to overcome some of said problems. The Meyer invention, in essence, provides a low operational pressure equivalent to a micronebulizer system at the lower aspect thereof, and also provides a series of impactors thereabove in a torch system portion of the invention. The Meyer invention provides greater stability in both construction and in nebulized sample solution flow to an ICP. Said impactors serve to deflect large diameter droplets (e.g. over approximately fifteen (15) microns in diameter), and prevent their ejection from the upper aspect of the invention, and in addition to buffer the ejected flow of nebulized sample solution.
In view of the benefits provided by the Fassel et al. micro nebulizer, and in view of the difficulties associated with use thereof, which difficulties have received recognition from users thereof, there is thus demonstrated a need for an improved direct injection micro nebulizer.
Continuing, sample preparation for introduction to a sample analysis system typically involves not only a sample solution nebulization step, but also sample desolvation and solvent removal steps. Nebulized sample solution droplets are typically desolvated prior to being entered, for instance, to an ICP or MS. If this is not done, plasma instability and spectra emission interference can occur in plasma based analysis systems, and solvent outgassing in MS systems can cause pressures therein to rise to unacceptable levels.
Desolvation of sample solution droplets involves two processes. First, sample solution droplets are heated to vaporize solvent present and provide a mixture of solvent vapor and nebulized sample particles; and second, the solvent vapor is removed. The most common approach to removing solvent is by use of low temperature condenser systems. Briefly, in said low temperature condenser systems the nebulized sample solution droplets are heated to vaporize the solvent present, and then the resulting mixture of solvent vapor and nebulized sample particles is passed through a low temperature solvent removal system condenser. When the solvent present is water very high desolvation efficiency, (e.g. ninty-nine (99%) percent), is typically achieved, when the solvent condensing temperature is set to zero (0) to minus-five (-5) degrees centigrade. However, when organic solvents are present the desolvation efficiency at the indicated temperatures is typically reduced to less than fifty (50%) percent. Use of lower temperatures, (e.g. minus-seventy (-70) degrees centigrade), can improve the solvent removal efficiency, but will also cause greater loss of nebulized sample particles as an undesirable accompanying effect. In addition, low temperature desolvation systems typically comprise a relatively large volume condenser. This leads to sample "carry-over" problems from one analysis procedure to a subsequent analysis procedure as it is difficult to fully flush out the relatively large volume between analysis procedures.
A Patent to D'Silva, No. 5,033,541 describes a high efficiency double pass tandem cooling aerosol condenser desolvation system which has been successfully used to desolvate ultrasonically nebulized sample droplets. This invention presents a relatively small internal condenser volume, hence minimizes sample carry-over problems, however, while the invention operates at high desolvation efficiencies when water is the solvent involved, it still operates at lower desolvation efficiencies when organic solvents are used. The invention also requires sample passing therethrough to undergo turbulance creating direction reversals, and the use of relatively expensive refrigeration equipments. Turbulance in a nebulized sample flow path can cause reagglomeration of nebulized sample solution droplets and, especially when very low temperatures are present, recapture of nebulized desolvated sample particles present.
A Pat. to Skarstrom et al., No. 3,735,558 describes a counter-flow hollow tube(s) enclosed filter, mixed fluids key component removal system. Briefly, the invention operates to cause separation of key components from mixed fluids, such as water vapor from air, by entering the mixed fluid at one end of a single, or a series of, hollow tube(s), the walls of which are selectively permeable to the key components of the mixed fluid which are to be removed. A gas is entered to the system at the opposite end of the hollow tube(s), which gas is caused to flow over the outside of the hollow tube(s) in a direction counter to that of the mixed fluids, to provide an external purge of the key components of the mixed fluid which diffuse across the hollow tube(s). Diffusion of key components is driven by pressure and concentration gradients across the hollow tube(s). This approach to removal of diffusing components does not require the presence of low temperature producing refrigeration equipments, and presents a relatively small internal volume.
Two Pats. to Vestal, Nos. 4,958,529 and 4,883,958 also describe systems which utilize counter-flow enclosed filters systems, with the application being to remove solvent vapor from nebulized samples produced by a spraying technique. The vestal Patents state that the properties of the filter material used are not critical to the operation of the invention, but suggest the use of filter material available under the tradename of ZITEX. Said filter material provides a pore size of from two (2) to five (5) microns with a corresponding porosity of up to sixty (60%) percent. ZITEX is typically available in sheet form and enclosed filters made therefrom are typically constructed from a multiplicity of spacers and two sheets thereof. To provide an enclosed filter which is sufficiently long to provide reliable solvent vapor removal, in a reasonable space, it is typically necessary to arrange the spacers in a pattern which requires many severe sample flow path direction changes. A flow of solvent vapor and nebulized sample particles passing through such a tortuous pathway experiences turbulance. Turbulance causes sample to adhere and accumulate inside the enclosed filter thereby causing sample carry-over problems. The Vestal Patents also describe the heating of the enclosed filter to further assure continuous vaporization of solvent vapor present therein, and the flow of a gas outside the enclosed filter to remove solvent which diffuses through the enclosed filter.
The above presentation shows that the preparation of liquid samples for analysis in gas phase or particle analysis systems typically involves:
In view of the above it can be concluded that a sample introduction system which at once: provides high sample solution nebulization efficiency and aerosol conversion rate; provides more efficient, (e.g. more than ninty-nine and nine-tenths (99.9%) percent), desolvation of the produced nebulized sample solution droplets in a manner which is equally successful whether water or organic solvents are present; minimizes sample carry-over by increasing sample transport efficiency therethrough and which optimizes system long term operational stability, would be of great utility. Such a sample introduction system is taught by the present invention.